1. Field of the Invention
The present invention relates to a method for carrying out a non-destructive inspection on a semiconductor chip in a wafer state, in an installation state, etc. in a production process, and more specifically to a method for detecting or inspecting a portion having a leakage including a short circuit, increasing resistance, or a disconnection.
2. Description of the Prior Art
Conventionally, such a non-destructive inspection technique has been used to detect a defective portion of a p-n junction in a non-destructive manner as part of analysis of failure and defect in a semiconductor chip.
FIG. 15 illustrates the principle of the conventional non-destructive inspection method. When a laser beam 2 is radiated onto a p-n junction 1, a pair of an electron 3 and a positive hole 4 is generated. Each of them flows in the opposite direction by the electric field of the empty layer of the p-n junction 1 and the electric field by an external power source 5. Thus, the flowing current is referred to as a current by an OBIC (optical beam inducted current) phenomenon. This OBIC current 6 is detected as a current or a variation of a current by an ammeter 7 connected in series to the p-n junction 1. FIG. 16 illustrates an example of the conventional technology of detecting a defect by an OBIC current. It shows a defect 18 promoting the recombination on the p-n junction 1 with the same configuration as in FIG. 15. When a laser beam is radiated onto a non-defect portion like a laser beam 22, an OBIC current flows just as in the case shown in FIG. 15. On the other hand, when a laser beam is radiated onto the defect 18 promoting the recombination like a laser beam 22, the recombination annihilates a pair of an electron and a positive hole if it is generated, and no OBIC current flows. Thus, the position of the defect promoting the recombination can be specified.
The OBIC phenomenon at the p-n junction is, as disclosed in Japanese Patent Publication No. 10-135413, not only used to detect a defect of the p-n junction, but also used to detect a disconnected wire in the wiring. The method is described below by referring to the side view shown in FIG. 17 and the plan view shown in FIG. 18. P-n junctions 1001, 1002, and 1003 are connected in series. The wiring is formed parallel to each of the p-n junctions. When the wiring is disconnected by a disconnection defect 1028, an OBIC current different from the currents of the other p-n junctions flows the p-n junction 1002 connected parallel to the disconnected wiring when a laser beam is radiated, thereby successfully specifying the disconnect wiring.
There is another conventional technology. As disclosed by Beyer, J. et al., Applied Physics Letter (Appl. Phys. Lett.) vol. 74, No. 19. pp. 2863-2865 (1999), a semiconductor substrate (hereinafter referred to as a raw wafer) before configuring an element as a semiconductor device is used in carrying out a non-destructive inspection to inspect the non-uniformity of the impurity density of a semiconductor substrate. FIG. 19 shows a basic configuration. When the laser beam 2 is radiated onto a raw wafer 200, a pair of the electron 3 and the positive hole 4 occurs. The pair of the electron 3 and the positive hole 4 is immediately recombined and annihilated if the impurity density in the raw wafer 200 is uniform. However, if the impurity density is not uniform, the OBIC current 6 flows. Magnetic flux 11 formed by the current is detected by a superconducting quantum interference device (hereinafter referred to as SQUID) fluxmeter 12.
There is the following problem with the above mentioned conventional technology.
In the first conventional technology, to first detect a current change, an electrical connection is required between the inspection device and a semiconductor chip, and an inspection can be carried out only after the completion of the preprocess of the production process of a semiconductor to be inspected, and after the completion of a bonding pad.
An inspection can be carried out after a bonding pad is completed, that is, after a postprocess is completed. However, in this case, there are a number of combinations for an electrical connection, and a large number of process steps and a high cost are required in preparation for the connection. The conventional technology is not effective if a currently defective portion is not electrically connected in series with an ammeter. Therefore, to carry out the inspection without fail, it is necessary to electrically connect an ammeter to all bonding pads capable of passing an OBIC current. Normally, the flow of the OBIC current is detected between two terminals as shown in FIG. 16. However, the number of combinations of two terminals increases substantially in proportion to the square of the number of bonding pads. Therefore, when the number of bonding pads increases, the number of combinations largely increases. To prepare for the connections each time the type of object chips changes, it is necessary to prepare for an exclusive jig and change the connection, thereby requiring a number of process steps and a high cost.
Furthermore, as described above, in addition to the increasing number of combinations of connections, the electrical connections of the terminals to other devices and parts also affect an inspection, thereby causing the problem that the interpretation of an observation result becomes complicated. Furthermore, the possibility that an inspection may deteriorate other devices and parts makes it considerably hard to actually carry out the inspection after the installation is completed.
The problem with the second conventional technology is that it is very hard to apply the technology as is to a semiconductor chip in view of the response speed. In the Applied Physics letter (Appl, Phys. Lett.) by Beyer, J. et al., vol. 74, No. 19, pp. 2863-2865 (1999) which is referred to as the second conventional technology in the Reference 2, an observation target is the OBIC current for a raw wafer, and the time constant is no more than 50 xcexcs, which is described as an observation result on page 2865 in line 4.
On the other hand, the attenuation of the OBIC current transitionally generated in a semiconductor chip proceeds exceedingly fast in most cases as compared with 50 xcexcs unless the current is led outside. The reason why the attenuation of the OBIC current transitionally generated in a semiconductor chip proceeds exceedingly fast in many cases is that the structures of the element in a semiconductor chip and the wiring are designed to be able to be operated at a high speed in many cases. Practically, a CR time constant which depends on the values of a capacitance C and a resistor R is designed to induce the maximum performance of the semiconductor chip in many cases. Therefore, the OBIC current generated in the semiconductor chip is often attenuated with the time constant. When a semiconductor chip operates at, for example, 1 GHz, the time constant has to be higher than 1 ns. To detect an OBIC current attenuating faster than 1 ns, the response frequency of the SQUID fluxmeter has to be higher than 1 GHz. From an economical viewpoint, the currently available SQUID fluxmeter cannot detect the magnetic flux. For example, the response frequency of the currently most practical high-temperature superconducting DC-SQUID fluxmeter is approximately 1 MHz at most.
Described above are the problems with the conventional technology based on which the present invention has been developed. The problems from the viewpoint of needs are described below.
In the flow of producing a semiconductor device in a wafer process and sending it onto the market, a wafer proving test carried out after forming a bonding pad at the final stage of the wafer process is a method of determining whether or not a chip unit is acceptable in the conventional inspection methods. However, it is difficult to make an appropriate development and production plan by obtaining the yield a this late stage. Therefore, various monitoring processes are performed in the wafer process to predict the yield. The currently most attractive and practical method is a method referred to as a pattern defect inspection method, a method for inspecting a foreign substance and a defect, etc. (hereinafter referred to as a pattern defect inspection method) In this method, the size, form, frequency, distribution, etc. of a defect and a foreign substance can be informed of using the reflection and the scattering of a radiated laser beam, and the emission of a secondary electron and a reflected electron of a radiated laser beam. The obtained information is used in monitoring the state of the wafer process, improving the process, and predicting the yield. However, the pattern defect inspection method has a demerit based on its principle. That is, in this method, observations are not associated with electric characteristics of a transistor, wiring, etc. configuring a device. Namely, only a physically foreign substance and an abnormal shape are observed. Therefore, the determination as to whether or not a finished device chip is acceptable is only indirect determination.
The present invention aims at providing a new inspection method by overcoming the limit in applicable field, performance, etc. of the conventional non-destructive inspection method and apparatus for a semiconductor chip, improving the productivity and reliability of a semiconductor chip.
The non-destructive inspection method according to the present invention includes: a first step of generating a laser light ranging in wavelength from 300 nm to 1,200 nm, and generating a laser beam converging into a predetermined beam diameter; a second step of predetermined electrical connection means configuring of a predetermined current path for passing an OBIC current generated by an OBIC phenomenon when the laser beam is radiated onto the p-n junction and the vicinity of the p-n junction formed in the semiconductor chip to be inspected at least in the substrate including a wafer state and an installation state during the production process; a third step of scanning a predetermined area of a semiconductor chip while radiating the laser beam; a fourth step of magnetic flux detection means detecting magnetic flux induced by an OBIC current generated by the laser beam at each radiation point scanned in the third step; and a fifth step of determining whether or not there is a resistance increase defect including a disconnection defect or a leak defect including a short circuit defect in a current path.
At this time, a CR delay circuit comprising a capacitance C and a resistor R can also be included in the current path.
Furthermore, the electrical connection means can be designed as a conductive film applied to the entire top surface of the substrate of the semiconductor chip having at least one contact hole in a scattering layer area and having a p-n junction on the substrate.
It is also possible to set the fifth step such that if magnetic flux detected in the fourth step is equal to or larger than a predetermined value at a radiation point where no current path is configured for an OBIC current in a normal state, then it is determined that a leak defect including a short circuit defect has occurred in the current path containing the radiation point, and if magnetic flux detected in the fourth step is smaller than a predetermined value at a radiation point where a current path is configured for an OBIC current in a normal state, then it is determined that a resistance increase defect including a disconnection defect has occurred in the current path containing the radiation point.
As described above, the non-destructive inspection method according to the present invention is based on that an OBIC current generated by radiating a laser beam onto a p-n junction flows through the short circuit portion containing a leak defect as apart of the current path, and that the current induces magnetic flux. Furthermore, to use a SQUID fluxmeter which is a currently available high-sensitivity fluxmeter, it is necessary to have a configuration in which the attenuation time of an OBIC current is equal to or longer than 1 xcexcs, or the current is constant. Therefore, the current path is designed as a closed circuit or a CR delay circuit is inserted into the current path.
The basic configuration includes a laser beam (2 shown in FIGS. 1 and 2), a current path (600 shown in FIG. 1) through which a generated OBIC current flows, and a SQUID fluxmeter (12 shown in FIGS. 1 and 2) which is means for detecting induced magnetic flux. A resistor and a capacitance (670 and 660 shown in FIG. 2) for delaying a CR delay can be contained in the current path.
In an embodiment in a wafer state, means for generating a large amount of magnetic flux by passing a generated OBIC current through the longest possible current path can be configured in a wafer based on the configuration shown in FIGS. 1 and 2 (201 and 202 shown in FIGS. 3 and 4).
Furthermore, in an embodiment in an installation board (circuit substrate), another means for passing a generated OBIC current through the longest possible current path can be provided in a circuit substrate (402 shown in FIG. 6).
Normally, in an embodiment with a structure to be inspected for exclusive evaluation referred to as a test element group (hereinafter referred to simply as a TEG), the detection sensitivity can be improved by configuring another means for passing a generated OBIC current through the longest possible current path in a semiconductor chip to be inspected (603 shown in FIG. 9).
According to the present invention, not only a defect of a p-n junction is directly detected by the OBIC current generated by a p-n junction as a result of the radiation of a laser beam, a leak portion including a short circuit is detected in the current path formed by a short circuit of a portion electrically connected in series with the p-n junction or by a leak path using the flowing OBIC current. At this time, a non-touching observation can be made not by directly detecting an OBIC current, but by detecting the magnetic flux induced by the current. Furthermore, the magnetic flux generated by the OBIC current can also be easily detected by inserting a CR delay circuit containing a parasitic element in the current path.
Additionally, non-touching detection can be performed on a resistance increase defect including a disconnection defect based on the fact that an OBIC current decreases or does not flow due to a resistance increase defect including a disconnection defect in the current path.
For example, FIG. 21 shows an example of a graph indicating the dependence of an OBIC current value for the resistance value in the path through which the OBIC current confirmed by an experiment by the inventor of the present invention flows. To be more practical, the value an OBIC current obtained when a laser beam having a wavelength of 1064 nm is radiated onto a part of the p-n junction in the LSI produced in the normal LSI production process from the surface on which an element of the LSI chip is formed is measured by changing the value of the resistance connected in series with the p-n junction, and by showing a graph of the measurement result with the horizontal axis indicating a resistance value and the vertical axis indicating a current value. The horizontal and vertical axes have a logarithm scale. As shown in FIG. 21, when the resistance value in the current path through which an OBIC current flows increases, the current value of the OBIC current decreases. For example, the value of the OBIC current obtained when the resistance value in the path is 1 Mxcexa9 is smaller by 3 digits or more than the value obtained when the resistance value in the path is 100xcexa9. The value of the magnetic field induced by an electric current is proportional to the current value according to the Biot-Savart""s law. Therefore, a resistance increase defect including a disconnection defect in the OBIC current path connected in series with the p-n junction can be easily detected as a change of magnetic flux. In addition, when a current path is generated by a defect occurring where there is normally no OBIC current, and when the current path not only clearly indicates 100 xcexa9 as a short circuit, but also indicates 1 Mxcexa9 appropriately referred to as a leak, the current value (0.1 xcexcA) detectable as magnetic flux although it is very low makes it possible not only to detect a short circuit defect, but also to detect a leak defect.
In the state before forming a bonding pad, a resistance increase defect including a disconnection defect and a leak defect including a short circuit can be detected by detecting the magnetic flux induced by an OBIC current. Furthermore, after forming a bonding pad, the above mentioned defects can be detected without selecting a terminal. Furthermore, in an installation state on a circuit substance, the above mentioned defects can be detected on a semiconductor chip. Means for forming a current path through which an OBIC current flows or a CR delay circuit can be classified as follows based on some cases.
(1) In a process where a conductive film is applied to the entire top surface of the wafer, by using only the conductive film (210 shown in FIG. 20(a), and 212 shown in FIG. 20(b)), or by setting the same potential for two portions, that is, one end of the conductive film (201 shown in FIGS. 3 and 4) on the top surface of the wafer and the diagonally opposite end on the substrate (202 in FIGS. 3 and 4), a current path (a path indicated by 6 shown in FIGS. 3 and 4, or 261 or 263 shown in FIG. 20) through the substrate can be generated through the top surface of the wafer and the p-n junction in which a leak portion including a short circuit and an OBIC current are generated.
When a pad-formed wafer is used, a similar embodiment can be realized, the entire pad is short-circuited using silver paste or thin gold film or by short-circuiting the pad through a prover. However, in this case, the current path is complicated. Furthermore, a current path cannot be generated in many cases. Therefore, it is not so efficient as the above mentioned method.
When a diced or packaged chip is analyzed, a similar embodiment can be realized by assuming that the above mentioned wafer is a chip. That is, the entire surface of the chip is covered with conductive film such as silver paste, thin gold film, etc. by exposing the top surface of the chip or making a space between the chip and the packaging material. In addition, the chip substrate side can only be exposed at least to the potion where an electrical connection is required and the portion where laser radiation is required. In this method, as compared with the conventional technology, the cost and process steps of an electrical connection can be considerably reduced. Otherwise, all pins can be provided for a short-circuited socket. However, after a pad has been formed, a current path cannot be formed in many cases, which is not an efficient method as in the case where a wafer is used after a pad has been formed. When a packaging process is completed, it is necessary for a chip on the laser radiation side to be exposed. However, it is not necessary that a chip is exposed on the SQUID side.
(2) In a state in which a bare chip is installed on the circuit substrate, two end portions can be selected in several methods depending on the position of a defect on the circuit. For example, a current path, which includes long substrate wiring (402 shown in FIG. 6) on a circuit substrate and passes through a p-n junction and a leak portion including a short circuit in the chip by short-circuiting the power source wiring of a circuit substrate and the substrate potential of a chip in an appropriate position selected on the circuit substrate, can be generated.
(3) When a TEG is formed on a chip with a view to monitoring the state of the process of producing a semiconductor chip or selecting the optimum value of a design parameter or a process parameter, a current path and a CR time constant can be freely set. For example, a path round a scribing line along the circumference of a chip, and a path round inside the scribing line and outside of the bonding pad are current paths which are long and determined, and in which magnetic flux can be easily detected (603 shown in FIG. 9).
In the cases (1) and (2) above, not only the current path is formed by a short circuit and a constant current is detected, but also a transient current can be detected by delaying the transient current based on the response speed of a detector by inserting a resistor and a capacitance into the current path in series as shown in its basic configuration in FIG. 2. In this case, the capacitance and a resistor may not require an additional circuit if a parasitic capacitance, a parasitic resistor, and a floating capacitance can be appropriately used.
It is common with (1) and (2) above, but without any electrical connection to a pad or a substrate, a closed circuit inside a chip or a CR delay circuit can be configured at a certain level, and magnetic flux by an OBIC current can be detected. If a defect can be detected in this method, it is the most efficient method.